loader: update autoboot description and move to loader.conf.5

Document "NO" special value for the autoboot_delay and move the
description to loader.conf.5.

imp reworked some of the wording from danger's patch.

Reviewed by: imp
PR: 85128
Differential Revision: https://reviews.freebsd.org/D11887

(cherry picked from commit 71f6aea4150c66784cbad42c1e1ff908d909c2ec)